The present invention relates to analysis methods using gas chromatography and to associated analyzing devices.
In gas chromatography, a dosing device injects a mixture of substances to be analyzed (a sample) in the form of the shortest possible, sharply delineated sample slug at the inlet of a separation device (e.g. a separation column or a system of separation columns). The sample is then transported through the separation device by a carrier gas stream. Individual components of the sample interact in different ways with a separation substance in the separation device, so they are transported through the separation device at different rates and appear in succession at the outlet. Thus, different successive zones containing the separated components of the sample are formed from the single mixed zone of the initial sample slug. The width of these zones is greater than that of the sample slug because of the transport of the components through the separation device and their interaction with the separation substance. At the outlet of the separation device, the separated components of the sample are detected by a detector, which generates an analog detector signal (chromatogram) having peaks for each individual component separated. The width of each individual peak corresponds to the width of the zone of the respective component, while the height of the peak depends on the concentration of that component in the carrier gas. The time integral over each peak, i.e., the peak area, is proportional to the quantity of the respective sample component, and thus to the quantity of sample dosed, and to the concentration of the respective component in the substance mixture for analysis. Therefore, to be able to determine the quantities of individual sample components and/or their concentrations from the peak areas, the peak areas must be multiplied by calibration factors (response factors), which must be determined in advance by calibration measurements for each component on the basis of calibration samples having a predetermined quantity and composition.
Therefore, for accurate and reliable gas chromatographic analysis of a sample, the sample must be injected with a suitable precision and reliability. By calibrating the chromatograph at certain intervals, it is possible to detect changes in the chromatograph since the last calibration. If substantial changes have occurred, it must be feared that the analytical results obtained since the last calibration might have been faulty and might have resulted in faulty process control, for example. In such cases, the calibration cycles can be shortened. Starting at the time of each new calibration, analytical results provided by that chromatograph are assumed to be correct until the next calibration.
However, validation of analytical results, i.e., confirmation of the accuracy of each individual analysis, is being required today to an increasing extent, but this cannot be accomplished through calibration alone, as shown above.
One possible method of validation is the so-called internal standardization method (100% method) in which a sample is analyzed completely quantitatively, and then the total quantity of sample is calculated from the sum of all component quantities determined from the peak areas. If the total calculated quantity of sample fluctuates in successive analyses, the areas of the peaks to be determined are corrected accordingly. However, the prerequisites for this include complete analysis of a sample, using only a single detector (because two or more detectors may vary to different extents) and constancy of the response factors of all components of the sample. However, these prerequisites are not usually met. Thus, in process chromatography in particular, components not of interest are usually cut out or backflushed at cutting points within the separation system, but detectors are not present at all outlets of the separation system.
Another possible method of validating analytical results is the internal standard method, in which a known amount of a standard compound is added to the sample. The areas of the peaks to be determined are corrected to the extent to which the peak area of the standard peak fluctuates in successive analyses. However, although this method is used in laboratory chromatography, it cannot be used in process chromatography in general, because accurate addition of a standard to a sample taken online from a process creates more problems than it eliminates.